C.J. Barnes

The influence of heterocyclic thiols on the electrodeposition of Cu on Au(111)

Abstract We report the influence of two structurally related heterocyclic thiols, 2-mercaptobenzothiazole and 2-mercaptobenzimidazole, on the underpotential (upd) and bulk deposition of Cu on Au(111). X-ray photoelectron spectroscopy analysis of the organic adlayers reveal that after self-assembly from the liquid phase, the primary adsorbate species is consistent with a thiolate. The Au(111)-(√3 × 22) reconstruction is lifted to the (1 × 1) phase upon thiol self-assembly. At submonolayer thiol coverages, a new upd feature is observed, with an intensity that goes through a maximum at intermediate thiol coverages. This feature is consistent with Cu adsorption in the vicinity of chemisorbed thiol. A thiol coverage-dependent study of Cu upd suggests formation of small two-dimensional thiol islands. At saturation coverage, the adsorbates passivate the surface towards Cu upd and significantly hinder bulk processes. Investigation of a previously reported potential-dependent reorientation of 2-mercaptobenzothiazole disputes a structural mechanism of ring ‘opening’ and ‘closing’ of the thiol layer. Instead, we suggest that instability of the thiol during cycles of Au oxide formation and reduction results in irreversible desorption of the thiol and complete restoration of Cu upd.

Molecular beam studies of CO oxidation and CO-NO reactions on a supported Pd catalyst

Abstract The surface reactivity of highly dispersed (85%) palladium cluster catalysts supported on γ-alumina (1.5 wt% loading) had been probed using the molecular beam scattering technique. This work illustrates the utility of molecular beam sources to study reaction kinetics on highly dispersed catalyst surfaces under UHV conditions. While CO adsorbs and desorbs molecularly throughout the coverage range, the Pd particles have been found to be extremely active with respect to NO dissociation forming a “mixed” layer of molecularly bound NO and atomic nitrogen and oxygen. Reaction kinetics of CO oxidation has been studied as a function of surface temperature by reaction of a pre-adsorbed oxygen layer with a CO molecular beam. A maximum reaction rate and percentage conversion to CO 2 was found to occur between 570 and 620 K. In contrast to previous studies of model Pd cluster catalysts, the highly dispersed Pd clusters have been found to be active for the CO/NO reaction probed by reaction of a CO beam with a NO pre-adsorbed layer.

The growth mechanism and structure of ultrathin cobalt films deposited on the Pd(111) surface

Abstract Ultrathin films of cobalt were deposited by thermal evaporation on the Pd(111) surface at room temperature. The combination of low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS) shows that cobalt grows forming multi-atomic layer islands. X-ray photoelectron diffraction (XPD) and low energy electron diffraction (LEED) indicate that for the first 1–2 atomic layers the cobalt islands adopt an fcc structure with the same in-plane parameter as the substrate. For thicker films, cobalt maintains the fcc stacking sequence with a gradual relaxation to the lattice parameter of bulk Co.

A re-interpretation of the Cu{100}/Sn surface phase diagram

The coverage dependent structural phase transitions of Sn on Cu{100} have been re-examined by low energy electron diffraction (LEED). Double scattering LEED pattern simulations have been applied both to a range of possible new models and to previously suggested structures with the aim of identifying the most likely surface geometries throughout the sub-monolayer coverage regime. A model consistent with both the Sn surface coverage and the complex split beam LEED pattern observed has been suggested for the low coverage (θ Sn = 0.21 ML) ordered phase based on a p(2 x 2) structure with light antiphase domain walls. We also demonstrate that higher coverage p(2 x 6) (θ Sn = 0.37 ML) and p(3√2 x √2)R45° (θ Sn = 0.50 ML) structures based on c(2 x 2) local periodicity yield a consistent explanation of the LEED data. While the simulations identify likely structures, the limitations of this approach mitigate against definitive structural assignments. However simulations for models based on c(2 x 2) structures incorporating defects in the form of periodic density modulations combined with substrate reconstruction lead to an enhanced agreement with observed LEED data compared to overlayer models previously suggested.

Surface structural transitions induced by repetitive underpotential deposition of Ag on Au(111)

Abstract The underpotential deposition (upd) of Ag on a Au(111) electrode in aqueous sulphuric acid has been studied by electrochemical scanning tunneling microscopy (ECSTM). The Au(111) surface morphology is substantially altered during repeated cycles in the Ag upd region. Prior to Ag upd, the surface consists of large, flat terraces (50–200 nm in width) with monatomic and diatomic steps. After several cycles of Ag deposition and dissolution, the surface is characterised by the appearance and growth of a high density of monolayer deep ‘pits.’ As cycling is continued, the pits expand rapidly to leave isolated Au islands accompanied by receding terrace edges. This process is followed by a slow lateral growth of Au islands which apparently fuse with terrace edges or aggregate with each other to form extended, atomically flat terraces. The surface topography is dominated by straight-edged, narrow terraces (15–50 nm in width) running parallel to or crossing each other to form a mesh, in which the terrace edges meet at an angle of 120 or 60°, characteristic of a (111) close-packed plane. The slow transformation of a high quality Au(111) surface in comparison to the faster transformation of a defect rich surface suggests that the kinetics of the transition process are related to the initial number of surface defects. The surface morphological changes accompanying Ag upd cycling are reflected in the cyclic voltammograms via dramatic changes in both peak shape and potential. The effect of variation of the supporting anion on this transition has also been investigated. Repetitive cycling of Ag upd on the Au(111) electrode in the presence of perchlorate anions results in similar alterations in voltammetric peak shape and potential. This weakly adsorbing anion appears to enhance the rate of structural transformation of the Au(111) surface.

An STM study of structural transitions during the nucleation and growth of Pd and Cu cluster catalysts on HOPG

Abstract We report a scanning tunneling microscopy investigation of the morphology of model metal cluster catalysts deposited on highly oriented pyrolytic graphite. Coverages have been determined from a quartz crystal oscillator which has been calibrated in absolute terms using anodic stripping voltammetry. Both metals (Pd and Cu) show marked structural transitions as a function of the coverage. At high coverages (> 50 monolayers) an essentially continuous metal film is formed. However, the film is granular in nature consisting of tightly packed spherical clusters which have a surprisingly narrow size distribution. Cu clusters have a most probable diameter of 7.5 nm with a distribution of ±2.5 nm and Pd clusters are smaller having a most probable diameter of 4.2 nm with a distribution ranging from 3 to 6.4 nm. The cluster size is coverage independent down to the lowest coverages imaged which are of monolayer thickness. At intermediate coverages (4 to 8 monolayers) large areas of unadorned graphite are present, the clusters are nucleated in rafts in which a large number of clusters are aggregated both on the atomically flat terraces and at defect sites such as atomic and cleavage steps. At low coverages (≤ 3 monolayers) the clusters are located almost entirely at defect sites. No Ostwald-Ripening has been observed at 300 K. Time dependent morphological changes have been observed, indicating Cu aggregates appear to be highly mobile at room temperature. Aggregates are in continuous motion involving fission and re-attachment with a tendency for concentration of clusters along steps. This study illustrates that the exact morphology observed is a function of when the imaging occurs after growth.

Modification of formate stability by alloying: the Cu(100)-c(2 × 2)-Pt system

Room temperature deposition of Pt on Cu(100) has been studied by LEED, AES and desorption spectroscopy indicating that sub-monolayer Pt growth at 300 K leads to formation of a poorly ordered c(2 × 2) surface alloy co-adsorbed with Pt microclusters. In contrast to the Cu(100)-c(2 × 2)-Pd system, a high degree of crystalline perfection requires thermal activation which leads to surface Pt atoms switching to second layer sub-surface sites to form a c(2 × 2) underlayer below a copper monolayer. The presence of Pt in the second layer leads to electronic perturbation of the outermost copper monolayer. Formic acid adsorption on the Cu(100)-c(2 × 2)-Pt sub-surface alloy leads to formation of a formate intermediate with reduced stability relative to clean Cu(100) signalled by a ~30 K downward shift in the simultaneous CO2/H2 evolution. The formate decomposition on the Cu(100)-c(2 × 2)-CuPt underlayer follows first order decomposition kinetics and the decomposition activation energy is reduced from 119 kJ mol-1 (Cu(100)) to 110 kJ mol-1 on the CuPt underlayer alloy. Thicker Pt films with Pt coverages of 1 ML up to 2.5 ML containing significant quantities of Pt in the outermost layer in a local c(2 × 2) environment lead to an additional downward shift in the formate decomposition temperature to 406 K corresponding to a decomposition activation energy of 104 kJ mol-1. This activation energy corresponds to the formate stability on a mixed CuPt bimetallic site. Thus, the surface chemistry of Pt in Cu3Pt alloys differs significantly from that of pure Pt resulting in a much increased stability for the formate intermediate.

LEED investigation of the alloying/de-alloying transition in the Cu{100}/Bi system

The surface structures formed by deposition of 0.25 ML and 0.50 ML Bi on Cu{100} at room temperature have been determined quantitatively using the symmetrised automated tensor low energy electron diffraction (SATLEED). At θBi = 0.25 ML, Bi forms a semi-ordered p(2 × 2) surface alloy with the Bi atoms located at a height of 0.56 ± 0.06 A with respect to centre of gravity of the buckled outermost Cu layer (buckling amplitude = 0.11 A). The interlayer spacings in the first three substrate layers are found to be: d12 = 1.71 ± 0.06 A, d23 = 1.82 ± 0.06 A, d34 = 1.81 ± 0.06 A (dbulk = 1.807 A). The first and the third Cu layers are found to be buckled by 0.11 ± 0.06 A and 0.05 ± 0.06 A, respectively. At higher Bi coverage, de-alloying occurs culminating in formation of a well ordered c(2 × 2) overlayer at θBi = 0.50 ML. Bi atoms occupy the four-fold hollow sites with a vertical Bi–Cu interlayer separation of dBi–Cu = 2.17 ± 0.06 A above a slightly perturbed substrate. The interlayer spacing in the first four substrate layers are found to be: d12 = 1.82 ± 0.03 A, d23 = 1.80 ± 0.03 A and d34 = 1.84 ± 0.03 A. A small buckling of 0.02 ± 0.02 A is detected in the second Cu layer in which Cu atoms below Bi atoms are rippled outwards. The structures obtained by LEED are compared to those evaluated recently for the same system by surface X-ray diffraction.

A SATLEED study of the geometric structure of Cu{100}-Pd monolayer surface alloys

Abstract The structure of a Cu {1 0 0} -p(2×2) surface alloy formed by deposition of 1 ML of Pd on Cu {1 0 0} at room temperature has been studied by symmetrised automated tensor low energy electron diffraction. The favoured model from the wide range tested consists of a double layer ordered c(2×2) CuPd alloy with p(2×2)-p2gg symmetry introduced into the outermost layer via clock rotation of the CuPd monolayer with the corners of the p(2×2) unit cell centred over second layer Pd atoms ( R p =0.21). Lateral shifts of the top layer Cu and Pd atoms are determined to be 0.25±0.12 A. Substitution of 0.5 ML of Pd in both layers 1 and 2 leads to a significant expansion of the outermost two interlayer spacing to 1.93±0.02 A (+6.6±1.1%) and 1.90±0.03 A (+5.3±1.7%) and a rippling of Pd and Cu atoms in the outermost layer of 0.06±0.03 A with top layer Pd atoms rippled outwards. This model is in agreement with previous ion scattering studies of a Cu:Pd stoichiometry of 1:1 in the outermost two layers. A second mode of film growth consisting of adsorption of 0.5 ML of Pd on a copper capped Cu {1 0 0} -c(2×2)-Pd underlayer alloy leads to a structure which retains a simpler c(2×2) periodicity, suggesting that the growth of the p(2×2)-glide line phase requires a c(2×2) CuPd outermost template.

A tensor LEED study of an unusual cyclic hydrocarbon intermediate formed by benzene adsorption on Co(101̄0)

Abstract Tensor LEED has been used to determine the geometry of a p(3×1) phase formed by benzene adsorption to saturation coverage on Co (10 1 0) at 300 K. In contrast to benzene overlayers previously subjected to quantitative surface structural analysis, adsorption occurs with the ring in a tilted geometry across the substrate close packed [12 1 0] atomic rows in an off-centre bridge site with C s symmetry.